A galanin is a physiologically active peptide consisting of 29 amino acid residues found for the first time in a porcine small intestine extract [FEBS Lett., 164, pp 124-128 (1983)], and has been identified in various species other than porcine animals, such as mammalian, avian, reptilian and piscine animals. Those reported for their amino acid sequences are a human galanin (FEBS Lett., 283, pp 189-194 (1991)), a bovine galanin (FEBS Lett., 234, pp 400-406 (1988)), a rat galanin (J. Biol. Chem., 262, pp 16755-16758 (1987)), an ovine galanin (Peptides, 12, pp 855-859 (1991)) and the like, and the 15 amino acid residues from the N terminal are preserved among the species.
Known porcine galanins are a precursor protein consisting of 123 amino acid residues (preprogalanin (1-123); Proc. Natl. Acad. Sci. USA, 83, pp 6287-6291 (1986)), a precursor which is longer than a galanin by 9 residues at the N terminal, namely, preprogalanin (24-61) amide, and a preprogalanin (37-61) amide in which 4 residues at the N terminal of a galanin are deleted (Peptides, 13, pp 1055-1060 (1992)).
Known physiological activities of the galanins are an acetylcholin release inhibiting effect in a hippocampus (Brain Research, 709, pp 81-87 (1996)), a feeding center stimulating effect in a hypothalamus (Obesity Research, 3, pp 5735-5895 (1995)), a pituitary hormone release stimulating effect in a pituitary gland (Neuroscience Letter, 75, pp 49-54 (1987); Endocrinology, 134, pp 529-536 (1994); Peptides, 7, pp 51-53, (1986)), an insulin secretion inhibiting effect in a pancreas (Acta Physiol. Scand., 139, pp 591-596 (1990)) and the like, each of which is believed to be exerted via a galanin receptor.
The galanin receptors are classified into three subtypes (GALR1, GALR2, GALR3), and the genes have been cloned for GALR1 in humans, rats and mice (Proc. Natl. Acad. Sci. USA, 90, pp 3845-3849 (1993); J. Mol. Neurosci., 6. pp 33-41 (1995); FEBS Lett., 411, pp 225-230 (1997)), for GALR2 in rats (FEBS Lett., 405, pp 285-290 (1997); Mol. Pharmacol., 52, pp 337-343 (1997); J. Biol. Chem., 272, pp 24612-24616 (1997) and for GALR3 in rats (J. Biol. Chem., 272, pp 31949-31952 (1997). Each of these three galanin receptors has 7 hydrophobic regions (transmembrane domains) characteristic to a G protein-coupled receptor, and is considered to stimulate an intracellular transmission system via an activation of a G protein.
A galanin was proven to bind a galanin receptor of any of these three subtypes. The binding affinity of a galanin is the highest to GALR1, and then next highest to GALR2 and then GALR3, and the affinity to GALR3 is lower by about 10 times than to GALR1 (J. Biol. Chem., 272, 31949-31952, 1997). A galanin was reported also to induce a cAMP production inhibition in a GALR1-expressing cell (Proc. Natl. Acad. Sci., USA 90, 3845-3849, 1993), to induce a cAMP production inhibition in a GALR2-expressing cell (Mol. Pharmacol., 52, pp 337-343 (1997)), and to induce an enhanced inositol-phosphate metabolism and an increased intracellular calcium ion level (J. Biol. Chem., 272, 24612-24616, 1997).
The only intrinsic agonist to a galanin receptor identified so far is a galanin. There is no report of a utilization of an activating reaction of an agonist-dependent G protein (G protein-coupled receptor protein) in a galanin receptor, for example, 35S-labeled guanosine-5′-O-3-thiotriphosphate ([35S]GTPgS) binding increasing reaction (Methods in Enzymology, 237, pp 3-13 (1994)) or a GTP hydrolozation-promoting reaction (Methods in Enzymology, 237, 13-26, 1994) for the purpose of searching for a ligand of a galanin receptor.
It is desired to discover a novel intrinsic agonist which is different from a galanin in the selectivity for (specificity to) a subtype of the galanin receptors.